<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <script src="../lib/vue.js"></script>
    <title>Document</title>
</head>
<body>
    <div id="app">
        <input type="text" v-model="n1">
            
            <select v-model="opt">
                <option value="+">+</option>
                <option value="-">-</option>
                <option value="*">*</option>
                <option value="/">/</option>
            </select>
            
        <input type="text" v-model="n2">
            
        <input type="button" value="=" @click="calc">
            
        <input type="text" v-model="result">
    </div>
</body>
<script>
     var vm = new Vue({
      el: '#app',
      data: {
        n1: 0,
        n2: 0,
        result: 0,
        opt: '+'
      },
      methods: {
        calc() { 
            var codeStr = 'parseInt(this.n1) ' + this.opt + ' parseInt(this.n2)'
          this.result = eval(codeStr)
        }
      }
    });
</script>
</html>